KWin doesn't require XWayland, it provides XWayland. Wayland is actually just a protocol, which must be implemented in a compositor. It is different from Xorg where the X server and the compositor where two separate things. SailfishOS has its own Wayland compositor based on QtWayland which doesn't provide XWayland. Plasma Mobile uses KWin, which support both Wayland applications and X applications through XWayland. Porting Plasma mobile to Mer should be quite easy. They said their first builts were on Mer, but that they moved to Ubuntu because several people in their team were already familiar with Ubuntu's build system for working on Kubuntu.

edit: After reading this blog, it seems that I am wrong. There is a XWayland library, so it is not just a protocol. I think the fact SailfishOS doesn't support XWayland is still because of QtWayland, but don't quote me on this, things seems to be more complicated than I thought it was.